1. Field of the Invention
The present invention generally relates to methods and apparatuses for handling a moving object, such as a substrate, in a processing system. Specifically, the present invention relates to methods and apparatuses for determining the center of a substrate while the substrate is moving through a substrate handling environment of a vacuum processing system.
2. Background of the Related Art
Vacuum processing systems for processing 100 mm, 200 mm, 300 mm or other diameter substrates are generally known. Typically, such vacuum processing systems have a centralized transfer chamber mounted on a monolith platform. The transfer chamber is the center of activity for the movement of substrates being processed in the system. One or more process chambers mount on the transfer chamber at the position of slit valves through which substrates are passed by a substrate handler, or robot, pivotably mounted in the transfer chamber. Access to the transfer chamber from a clean ambient environment is typically through one or more load lock chambers attached at other slit valves. The transfer chamber substrate handler is mounted in the middle of the transfer chamber and can access each of the process chambers and load lock chambers to transfer a substrate therebetween. The load lock chambers may open to a very clean room, referred to as the white area, or to a substrate handling chamber, typically referred to as a mini-environment. The mini-environment transfers substrates in a very clean environment at atmospheric pressure from pods, or cassettes or carriers, seated on pod loaders to the load lock chambers.
The mini-environment has a substrate handler for transferring the substrates. The substrate handler in the mini-environment is typically different from that in the transfer chamber, since it is typically capable of translational and vertical movement as well as rotation and extension; whereas, the substrate handler in the transfer chamber is typically only capable of rotation and extension. Either type of substrate handler has an arm assembly for manipulating the substrates that it transfers. One prevalent type of arm assembly has multiple arms pivotably attached to each other at pivot joints in order to extend and retract a blade, or end effector, which supports the substrate. The position of the arms is typically determined from an encoder that detects the angle of the pivot joints. A controller, such as a microcomputer, receives signals from the encoder and calculates the position of the blade.
A typical processing system includes a substrate center-finding system locates the center of the substrate in order to adjust the location of the substrate, so the substrate is centered on each of the structures which support the substrate in the system to avoid damage to the substrate and to ensure proper processing of the substrate. The center-finding system typically includes a set of emitter/sensor pairs, such as infrared beam emitters and sensors, for detecting the edge of the substrate. The center-finding system is typically disposed in a part of the processing system through which the substrate is passed, such as the transfer chamber or the mini-environment, so that the sensors can detect the edge of the substrate at several locations as the substrate passes through the beams. These substrate center-finding systems that determine the location of the center of a substrate while the substrate is moving are called on-the-fly center-finding systems.
On-the-fly center-finding systems are quite often used to make corrections for substrate misalignment. They typically consist of banks of through-beam or reflective sensors, such as an infrared emitter/sensor pair, arranged in the chamber through which the substrates pass. A typical arrangement for the sensors may have three to nine sensors arranged in one to three sensor banks. A substrate handler passes a substrate through the sensor beams. When the substrate interferes with, or cuts, a beam, the associated sensor is triggered and sends a signal to the controller indicating the trigger. When the controller receives a trigger signal, the controller records the encoder position. Thus, as the substrate is passed through the bank of sensors, information on the substrate position is obtained by recording the substrate handler encoder position every time a sensor triggers by and computing the substrate center from the positions. The accuracy and repeatability of existing methods, however, is dependent on the accuracy of placement and alignment of the sensors, the need to move the robot along straight line or circular paths and a known, constant speed of the substrate. For example, some types of on-the-fly center-finding systems require the substrate to be moved in a straight line through the sensor beams, use two sensors that must be arranged in a line perpendicular to the straight line of the substrate movement, assume the radius of the substrate, and do not work for substrates having substrate flats. Another type of center-finding system requires the substrate to be moved in a circular arc and uses three sensors that must be arranged in a line perpendicular to a tangent of the arc. Some centering-finding systems also require the use of a calibration substrate to calibrate the system, so the performance of the system is also dependent on the characteristics of the calibration substrate.
FIG. 1a shows an example of a substrate center-finding system such as the one described in U.S. Pat. No. 4,819,167, which is assigned in common with the present application and is incorporated herein by reference. In this center-finding system, a substrate 14, on a blade 16, is passed through an array of sensors 10-12 in a straight-line path in the direction of arrow A to determine the x-y coordinates of points on the edge of the substrate 14, with the x-axis being in the direction of the substrate path. The sensors 10-12 are required to be positioned in a straight line that is perpendicular to the path (arrow A) of the substrate 14 and blade 16. A further requirement is that the blade 16 has a hole 18, which must be aligned with the middle sensor 11. The location of the blade 16 is calibrated by sending a calibration substrate through the center-finding system to find the center of the blade 16. Other prior art center-finding systems may also require the substrate to be moved in a straight line as described in this example, or may require that the substrate be moved in a circular line.
The center-finding system shown in FIG. 1a requires seven coordinate points, one for the blade position, and six for the substrate position. As the substrate 14 and blade 16 pass through the sensors 10-12, the blade 16 triggers the middle sensor 11 when the hole 18 is detected at point X.sub.1, thus providing the blade position. For some substrate center-finding systems of this type, the hole 18 may be very difficult to align to the sensor 11 and, thus, hard to detect. As the substrate 14 continues to move, the substrate 14 triggers the middle sensor 11 at the leading edge of the substrate 14, point X.sub.3. The substrate 14 triggers the outer sensors 10, 12 to provide the next leading edge positions at points X.sub.2 and X.sub.4. The substrate 14 next triggers the outer sensors 10, 12 to provide the trailing edge positions at points X.sub.5 and X.sub.7. Finally, the substrate 14 triggers the middle sensor 11 to provide the last trailing edge position at point X.sub.6.
This center-finding system determines the x-coordinates for the points X.sub.2 -X.sub.7 by recording the distance the blade 16 has traveled at each of these points from point X.sub.1. The point X.sub.1, the blade position, is defined as the origin of the coordinate system, and the x-coordinate for the six substrate points are calculated with reference to the blade position. The average x-coordinate for each pair of substrate points for each sensor is calculated. If the substrate is completely circular, then all three averages should be about the same and should correspond to the x-coordinate for the center of the substrate 14. However, most substrates have a substrate flat 20 or notch in order to indicate the alignment of the substrate. In FIG. 1a, the substrate flat 20 is shown to be in the path of the middle sensor 11, so the substrate 14 will trigger the middle sensor 11 to provide the trailing edge point X.sub.6 at the substrate flat 20. The substrate points X.sub.3, X6 for this sensor 11 will provide an average x-coordinate that is substantially different from the averages for the other pairs of substrate points, thus identifying the fact that the middle sensor 11 was triggered by the substrate flat 20. The data points X.sub.3 and X.sub.6 for the sensor 11 can be discarded, and the other four data points X.sub.2, X.sub.4, X.sub.5 and X.sub.7 can be defined as valid, or useable, data points. The x-coordinate of the substrate center is determined from the average of the x-coordinates for the valid data points.
The y-coordinates of the sensors are predetermined from a measurement of the distance between the sensors and define the y-coordinates for each of the data points. A calculation for approximating the y-coordinate of the center of the substrate 14, is based on the x-y coordinates of the valid substrate points and the x-coordinate determined above. The correction for the center of the substrate is determined by comparing the calculated actual x-y coordinates of the substrate center with a calibrated x-y center coordinate corresponding to the blade center. Once the x-y coordinates for the center of the substrate 14 are determined, the controller for the substrate handler adjusts the movement of the substrate 14 to center the substrate 14 appropriately in a chamber where it is to be processed.
A problem with this center-finding system is that it has several inflexible requirements. One of the requirements of this system is that the substrate 14 must move in a straight line in the direction of arrow A. If the substrate 14 does not move in a straight line, then the data points X.sub.1 -X.sub.7 will not line up according to the dashed lines shown in FIG. 1a, and none of the calculations will be correct. Another requirement is that the sensors 10-12 must be in a straight line. If any of the sensors 10-12 is not in a straight line with the other two, then the calculation for the average x-coordinate, as measured from point X.sub.1, of the pair of data points for that sensor will not be valid. Yet another requirement is that the line of the sensors 10-12 must be perpendicular to the line of the substrate movement (arrow A). If the line of the sensors 10-12 is not perpendicular to the line of the substrate movement, then none of the average x-coordinates for each of the pairs of data points for all of the sensors 10-12 will be the same. Another requirement is that there must be at least three sensors 10-12. If there is less than three sensors, then it will not be possible to discard one pair of data points to account for the substrate flat 20, since the calculations cannot proceed with just one pair of data points. Still another requirement of this center-finding system is that, if the system starts processing a different size substrate, then the distance between the sensors 10-12 may have to be changed. If the processing system starts processing larger substrates, then the sensors may have to be spaced further apart, so the substrate flat cannot cross more than one sensor. On the other hand, if the processing system starts processing smaller substrates, then the sensors may have to be spaced closer together, so the sensors will not miss the substrate. Such repositioning of the sensors 10-12 requires very careful measurements of the new locations of the sensors. Each of these requirements adds to either the complexity of the overall system or its cost or both.
FIG. 1b shows an example of another substrate center-finding system that has certain inflexible requirements such as those discussed above. The substrate 40 is supported on a substrate blade 42 of a substrate handler disposed in a chamber, such as a transfer chamber. The blade 42 is operated by two struts 44 attached to a motion actuator of the substrate handler. The substrate handler can move the substrate 40 back and forth inside the transfer chamber in a circular path, represented by arrow F, in order to move the substrate 40 between the various process chambers and load lock chambers attached to the transfer chamber. A bank 48 of sensor units 50-52, for sensing the edges of the substrate 40, is mounted in the transfer chamber in the path of the substrate 40. The substrate 40 triggers the sensor units 50-52 on its leading and trailing edges as it passes through the sensor beams emitted from the sensor units 50-52. In addition to requiring that the substrate 40 be moved in a circular path, center-finding systems of this type have required that the sensor bank 48 include at least three sensor units 50-52, that the sensor units 50-52 be placed in a straight line, and that the straight line of the sensor units 50-52 be radial to the rotational center of the substrate handler. Thus, similar to the center-finding system shown in FIG. 1a, the type of center-finding system shown in FIG. 1b has inflexible requirements that add to either the complexity of the overall system or its cost or both.
Additionally, a problem with some prior art center-finding systems includes the need to have a calibrating substrate for an operator to calibrate the system. The calibrating substrate is known to have a very precise geometry, and the operator places it in the processing system with its centering and alignment known, so that the accuracy of the center-finding system may be determined.
A need, therefore, exists for a substrate center-finding system and method that permits a very flexible operation without strict requirements on the shape of the path of the substrate, the size of the substrate, the positioning of the sensors, the number of sensors, or a calibration substrate.